first of all, you need to ensure your solution use correct hostnames when you rendering that links.
Please check your site configuration :
<site name="website_1" hostName="www.site1.com" language="en" cacheHtml="false" virtualFolder="/" physicalFolder="/" rootPath="/sitecore/content/site1" startItem="/home" database="web" domain="extranet" allowDebug="true" htmlCacheSize="10MB" registryCacheSize="0" viewStateCacheSize="0" xslCacheSize="5MB" filteredItemsCacheSize="2MB" enablePreview="true" enableWebEdit="true" enableDebugger="true" disableClientData="false" />
<site name="website_2" hostName="www.site2.com" language="en" cacheHtml="false" virtualFolder="/" physicalFolder="/" rootPath="/sitecore/content/site2" startItem="/home" database="web" domain="extranet" allowDebug="true" htmlCacheSize="10MB" registryCacheSize="0" viewStateCacheSize="0" xslCacheSize="5MB" filteredItemsCacheSize="2MB" enablePreview="true" enableWebEdit="true" enableDebugger="true" disableClientData="false" />
if it's something like this.
Also please check if Rendering.SiteResolving is true on web.config.
<setting name="Rendering.SiteResolving" value="true" />
How you render your links on code ?
If you render with :
Sitecore.Links.UrlOptions defaultOptions = Sitecore.Links.UrlOptions.DefaultOptions;
string itemUrl = Sitecore.Links.LinkManager.GetItemUrl(item, defaultOptions);
please add:
defaultOptions.SiteResolving = true;